This invention relates to the field of microwave power amplifiers and more particularly to approaches for simultaneously providing both high efficiency and low third-order intermodulation (IM3) in microwave power amplifiers using Gallium Nitride High Electron Mobility Transistor (GaN HEMT) devices.
Power amplifiers are used to boost a small signal to a large signal and are highly frequency dependent. Solid state GaN amplifiers may be used in the higher frequency bandwidths. Power amplifiers generate a large amount of heat. Heat drops the gain and increases the noise figure of the amplifier. Heat also lowers amplifier efficiency.
Amplifier efficiency may be defined as the ratio of the output power delivered divided by the applied DC power which is called the drain added efficiency (DAE). It may also be defined as the ratio of the output power minus the input power divided by the applied DC power which is called the power added efficiency (PAE). Amplifier efficiency is improved if heat generation of the amplifier is controlled. Amplifier efficiency is also improved by providing proper impedances at the harmonic frequencies of the load and the source.
A set of parameters describing the scattering and reflection of traveling waves when a network is inserted into a transmission line are referred to as the scattering or S-parameters. The S-parameters are normally used to characterize high frequency networks, where simple models valid at lower frequencies cannot be applied. The S-parameters are normally measured as a function of frequency and are expressed as a complex gain including both a magnitude and a phase. For this reason, the S-parameters are often called complex scattering parameters. When the incident wave travels through the network, its value is multiplied (i.e. is gain and phase are changed) by the scattering parameter which stands for the gain of the network, thus yielding the resulting output value. So, when a wave travels through a network, the output value of the network is simply the value of the wave multiplied by the relevant S-parameter.
S-parameters can be considered as the gain of the network, and their subscripts denote the port numbers. The ratio of the output of port 2 to the incident wave on port 1 is designated S21. Likewise, for reflected waves, the signal comes in and out of the same port, hence the S-parameter for the input reflection is designated S11. For a two port network, S11 is the reflection coefficient of the input, S22 is the reflection coefficient of the output, S21 is the forward transmission gain, and S12 is the reverse transmission gain from output to input. The S-parameters are analogous to frequency response functions for high frequency circuits. The magnitude or gain of the S-parameters is expressed in units of dB or as a ratio and their phase is expressed in degrees or radians.
Most amplifiers tend to lose gain as input power level increases. This phenomenon is called gain compression. The ideal response of the device may be plotted as a straight line of a certain slope. When the actually measured output power is also plotted against the input power, the plot may stay linear at lower input power values but will start to deviate and fall with respect to the ideally linear response. Customarily, the input power at which the gain response of the device is reduced by 1 dB is defined as the compression point of the device or the one-dB compression point.
In order to obtain a high linearity power amplifier with good thermal management that would result in high efficiency, transistors with high compression points, thermal stability, and high frequency response must be used. The material properties of GaN make it a suitable candidate for high power microwave applications. GaN HEMT transistors have inherently high transconductances, which lead to better linearity, good thermal management, and high cutoff frequencies.
Power amplifiers are divided into classes such as class A, class B, class AB, class C, or classes D, E, and F. These classes do not refer to different types of transistors. Rather, changing the bias voltage of the same transistor changes its gain characteristics and therefore may change its class. In other words, the classes refer to biasing regions of the transistors forming the amplifier. Some amplifier classes provide good efficiency at the expense of linearity of response. Other classes have a highly linear response but are not highly efficient. For example, class B amplifiers have good efficiency but low linearity while class A amplifiers have good linearity and low efficiency. Class AB amplifiers represent a compromise between classes A and B. Class C has a very high efficiency but will also cause a large distortion in the signal being amplified.
During an input cycle, transistors biased near class B spend about the same amount of time turned off as they are turned on. This is regardless of input drive level. Transistors biased at class A are turned on for nearly the entire input cycle. Transistors biased at class AB are turned on somewhere between class A and class B or between 180 degrees and 360 degrees of the input cycle. There is no well-defined division between class A, class B, or class AB operation. So a class AB amplifier indicates a broad range. However, specifying the bias point using a fraction of the saturation drain current is more definite. For example a class B amplifier is biased at 0IDSS or with an idle current of times the saturation drain current IDSS and a class A amplifier is biased at 0.5IDSS. Then, the approximately 0.10IDSS is a bias point that is closer to class B than it is to class A while it can be generally referred to simply as class AB.
Nonlinear distortion in the output of a device that is characterized by appearance of frequencies that are linear combinations of the fundamental frequency present in the input signal and all the harmonics of the fundamental frequency is called intermodulation distortion. Intermodulation is referred to mixing of two signals of different frequencies to form additional signals that are not integer multiples, or harmonics, of either signal. Combining two signals of frequencies f1 and f2 into a signal of frequency, 2f1−f2 or 2f2−f1 is called third-order intermudulation (IM3). Intermodulation is not desirable because it causes nonlinearity in the output.
Because linearity in amplifier response comes at the cost of power output efficiency and vise versa, there is always a need for novel methods of improving both of these parameters simultaneously.
Further, amplifiers have been conventionally cascaded to increase efficiency. Some of the conventional cascading arrangements, however, promote intermodulation harmonics and contribute to distortion.
FIG. 1 shows a conventional arrangement of a two-stage amplifier 100 including two field effect transistor (FET) amplifiers 10, 20 that are coupled together in series. In the two-stage amplifier 100 a driver amplifier 10 precedes a power amplifier 20. A popular approach for achieving both high power and linearity simultaneously is to use pre-distortion techniques. In pre-distortion, the driver amplifier stage 10 pre-distorts the signal in such a way as to compensate for the distortion effects caused by the output power stage 20.
In the two-stage amplifier 100 of FIG. 1, the FET forming the driver stage 10 has a class B bias for high efficiency. This FET 10 is biased very close to pinch-off. As this FET 10 device is driven by increasing input power levels, the channel begins to conduct. When the input power level increases, the transconductance also increases leading to an increase in the device gain. The driver stage amplifier 10 also suffers from high distortion. The power stage 20 is usually a class AB amplifier that has low distortion but a low efficiency as well.
FIG. 2 shows a measured gain enhancement 30 for the driver stage amplifier 10 of the conventional two-stage amplifier 100 of FIG. 1. The gain expansion 30 corresponds to a driver stage amplifier 10 that is made from two GaN HEMT transistors each having a channel length of 75 μm, when the two transistors are coupled in parallel, biased near deep class AB (approximately at 0.10IDSS), and operating at 10 GHz frequency. The horizontal axis shows input power Pin in decibel-milliwatt (dBm) units, the vertical axes show the drain current Id in mA and the gain enhancement 30 in dB. A drain current curve 40 shows an increase in the drain current Id with increasing input power Pin. As the figure shows, as the input power Pin increases, the gain 30 expands almost 2 dB before it begins to compress.
Devices biased at class AB or A, corresponding to the second or power stage 20 of the conventional power amplifier 100, do not have gain expansion but will only compress at high enough input power levels. This phenomenon is also known as AM-AM distortion where a change in the amplitude of the input results in an undesirable distortion in the amplitude of the output. AM-PM distortion also occurs where a change in the input amplitude causes a distortion in the phase of the output. In gain compression the phase of the large signal S21 increases while in gain expansion, the opposite occurs, that is the phase of the large signal S21 decreases. Both of these distortion effects, AM-AM and AM-PM, are combined to offset the distortion in the final two-stage amplifier 100.
The results of driving a FET used in the power stage 20 and biased at class AB with a FET used in the driver stage 10 and biased near class B is an increase in RF efficiency and nearly flat gain over a wide range of input power Pin levels. Sizing and biasing the FET of the driver stage 10 such that it is near the peak of its gain expansion when the FET of the power stage 20, located in the final or output stage, is compressed for a given power level can accomplish this increased efficiency and flat gain. Further, the driver stage 10 FET may be sized to be larger than the size usually used for power amplifier design. Alternatively, the driver stage 10 FET may be biased at a higher drain to source voltage VDS. However, both of these techniques result in a decrease in amplifier efficiency.
FIG. 3 shows an IM3 variation of the two-stage amplifier 100 against an input power Pin to the amplifier. The input power Pin is shown on the horizontal axis in dBm units and the IM3 values 50 are shown on the vertical axis in decibel-cm (dBc) units. The FET devices are biased at 10 GHZ and at a VDS of 35 volts.
The resulting reduction in IM3 50 using gain compression followed by expansion is apparent in FIG. 3. The figure shows a valley or a “sweet spot” 60 where minimum IM3 50 occurs. This valley or sweet spot 60 for minimum IM3 is usually narrow.
Alternative amplifier arrangements are therefore needed that maximize gain while maintaining a low intermodulation distortion. Embodiments of this invention, disclose various approaches for simultaneously providing both high efficiency and low IM3 in a microwave power amplifier using GaN HEMT devices.